Continuous wave

From Academic Kids

A continuous wave (CW) is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off. Information is carried in the rhythm and spacing with which the signal is sent. CW is thus is a form of on-off keying (OOK).

Very early radio transmitters used a spark gap to produce radio-frequency oscillations in the transmitting antenna; these signals had a characteristic rapidly damped amplitude during each pulse of radiated energy. When alternators and later electronic oscillators became available, the signal strength remained constant during each code element, leading to the description of this technique as "continuous" waves.

An unmodulated carrier has no bandwidth and conveys no information; the act of keying the carrier on and off produces a finite bandwidth relating to the transmission rate. Strictly speaking, a keyed carrier may be referred to as "ICW" for "Interrupted continuous wave" but the necessity of keying is usually understood.

Continuous-wave radio was called radiotelegraphy because like the telegraph, it worked by means of a simple switch to transmit Morse code. However, instead of controlling the electricity in a cross-country wire, the switch controlled the power sent to a radio transmitter. This mode is still in common use by amateur radio operators due to its simplicity and reliability.

CW is the basis of the continuous-wave radar system, where a continuous wave is transmitted by one aerial while a second aerial receives the reflected radio energy. Also, continuous-wave lasers are lasers which operate in a non-pulsed mode.

In amateur radio parlance, the terms "CW" and "Morse code" are often used interchangeably, despite the distinctions between the two (Morse code may be sent using sound and light, for example).

References

Larry D. Wolfgang et. al, (ed), The ARRL Handbook for Radio Amateurs, Sixty-Eighth Edition , (1991), ARRL, Newington CT USA ISBN 0872591689

See also

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